1. Field of the Invention
The present invention relates to a sheet stacking device and an image forming apparatus including the same.
2. Description of the Related Art
A sheet stacking device that stacks sheets output from an image forming apparatus or the like is required to orderly stack the sheets at a predetermined position while keeping an eject order of the sheets, that is, required for a good stacking performance. To this end, a technique for arranging a sheet presser arm that prevents an ejected sheet from projecting downstream of an ejection direction in a sheet reception tray is known (see, for example, Japanese Unexamined Patent Publication No. SHO 64(1989)-8180).
If the number of sheet sizes is one, a shape, a position and the like of the sheet presser arm may be optimized according to the size. However, in a sheet stacking device dealing with a plurality of sizes of sheets, it is necessary to determine the shape and the position of the sheet presser arm so as to obtain the good stacking performance for the sheets of the plural sizes. Actually, however, it is difficult to uniformly obtain the good stacking performance for the sheets of all sizes. For example, in order to press a sheet having a smallest sheet length L1 among all available sizes by the arm, a free end of the arm should be arranged so as to suppress the sheet within the length L1 from exit rollers. In this case, a front end of a sheet having a largest sheet length L2 among all available sizes is pressed by the free end of the arm within the length L1 from the exit rollers. After the front end of the sheet is moved forward by a length equal to or larger than a length (L2−L1) in this state, a rear end of the sheet is apart from the exit rollers and the forward movement is stopped. Accordingly, if the stacking device has a larger difference between L1 and L2, i.e., the stacking device deals with sheets of sizes in a wider range, there is a greater difference in sheet stacking conditions according to the sizes. It is difficult to obtain a uniform action of the sheet presser arm. In this case, if the sheet of a large size is insufficiently stiff or a force of the arm that presses the sheet is excessively strong, then the sheet is wrinkled or smooth eject of the sheet is hampered and the forward movement of the front end of the sheet is often stopped. In order to avoid such a disadvantage, there is known, for example, a technique for providing a movable aligning member called “jogger” on a tray. This technique is intended to press ends of each ejected sheet by the jogger and align the sheets to one another. There is also known a technique for providing both the jogger and the sheet presser arm, and for using the sheet presser arm in an auxiliary manner to the jogger particularly for the sheet of a large size for which it is difficult for the jogger to obtain good stacking performance. Further, there is known, as another sheet aligning technique, a technique for inclining a sheet in one direction, providing a position restricting member in a lower portion of the inclined tray, and offsetting sheets stacked on an inclined tray toward the lower portion of the inclined tray by the action of gravity and aligning the sheets by the position restricting member.
Furthermore, there is a limit to the number of stackable sheets whether the number is small or large. It is, therefore, desirable that the stacking device includes a tray-full detection function or a tray-full prediction function that indicates a user to remove stacked sheets or stops ejecting sheets from the image forming apparatus if the ejected sheets are close to a full capacity of the tray. It is also desired to provide these functions by a simple configuration. To this end, there is known a technique for allowing one detection mechanism to serve as stacking height detection means of a plurality of independently elevatable sheet stacking means (see, for example, Japanese Unexamined Patent Publication No. 2000-177911).
The sheet presser arm according to the conventional techniques needs to press the sheets by an appropriate force to prevent the sheets from being unaligned in all stacked states from a state in which one sheet is stacked on the sheet exit tray to a full state in which sheets are full therein. In addition, it is necessary to set a sheet pressure force so as not to prevent movement of a newly ejected sheet. A mechanism of such a sheet presser arm tends to be complicated.
According to the conventional techniques, a range of sheet sizes in which the good stacking performance can be obtained by the sheet presser arm is restricted to a narrow range. If the jogger is used, it is necessary to provide a complicated aligning mechanism. Besides, since the user may possibly touch the moving member of the jogger, it is preferable to reduce a moving member as much as possible for safety reasons. The technique for inclining the sheet in one direction is simple in configuration. However, if sheets of a large size are stacked, a contact area between upper and lower sheets is large in stacked states and a frictional force that acts on the sheets is high. In order to align the sheets against this frictional force, it is required to set the gradient of the tray sharp. In order to secure the sharp gradient, excessive spaces are necessary below and above the tray, which is disadvantageously unsuited for a small-sized sheet tracker.
Meanwhile, a multifunctional machine having an external shape formed into sideway U-shape and stacking ejected sheets in a central space has become popular following a reduction in a size of the apparatus. Particularly in the multifunctional machine of this type, a sheet stacking device capable of ensuring a sufficient number of stacked sheets and a good stacking performance for the sheets ejected to a size-limited region is desired. In addition, a “movable job separator” that branches an eject destination into a plurality of destinations according to a plurality of printing modes such as a printer mode, a copier mode and a facsimile mode and that partitions the sheets is often installed in the multifunctional machine as the sheet stacking device.
According to the conventional technique, separate mechanisms are provided for the tray-full prediction or tray-full detection function and for the improvement of the stacking performance, respectively. However, as demand for reduction in size of the stacking device rises further strongly, it is desired to simplify and reduce a size of a structure for these functions. It is also desired that the structure can be applied even to the stacking device that includes a plurality of sheet stacking sections as shown in the movable job separator.